1. Field of the Invention
The present invention relates to a scanning electron microscope, and more particularly, to a scanning electron microscope realized to observe a test sample by detecting back-scattered electrons scattered and emitted from a surface of the test sample in the air without a vacuum chamber which is allowed to observe the test sample in a vacuum state.
2. Discussion of Related Art
In recent years, the packing density of semiconductors and display and part materials has drastically increased with the development of IT/NT technology. There has been an increasing demand for analyses using an electron microscope capable of analyzing such IT/NT technology, and a representative example is a scanning electron microscope (hereinafter referred to as ‘SEM’).
The SEM is observation equipment using a principle of observing a test sample by focusing an electron beam shot from an electron gun through electromagnetic lenses, shooting the focused electron beam to a predetermined micro-area of a surface of the test sample mounted on a test sample stage of a vacuum chamber to collect secondary electrons emitted from the surface of the test sample and filling the scanned area with monitor pixels. Here, the SEM is an image analysis device used to observe a surface of the test sample (several ten nanometers (nm)).
Meanwhile, when the electron beam shot from an electron gun of the SEM and focused in the form of high energy is incident on a surface of the test sample, primary electrons incident on the test sample escape to a ground wire. In this case, a situation in which the primary electrons accumulate in the test sample without escaping to the ground wire is referred to as a charge-up phenomenon, and often occurs on non-conductor test samples such as organic matters, biological samples, etc.
Such primary electrons form hole pairs inside the test sample, and thus a surface of the test sample is negatively charged. When the surface of the test sample is negatively charged, a yield of the secondary electrons emitted from the surface of the test sample may significantly increase, whereas images may become too bright or get wiggly due to high mutual repulsion between incident electrons and secondary electrons, which makes it difficult to obtain normal high-quality clear images.
Therefore, to minimize such a phenomenon, a conventionally applied method is used to coat a conductive material such as Au or carbon on a surface of a test sample to a thickness of several ten nanometers. That is, this is a method of treating a test sample by applying a potential difference between a cathode target (Au, Pt or carbon) and a test sample bar (an anode) under a plasma atmosphere and emitting an electron beam, with which a surface of the test sample coated using a coating machine used to thinly apply ions onto a surface of the test sample is irradiated, along a test sample holder and a stage connected to the ground.
However, such a method of coating a test sample has a problem in that a liquid material of a liquid test sample such as a gel is not easily coated when the liquid test sample is applied. Also, a method of coating a liquid test sample after drying the liquid test sample also has a problem in that it is very difficult to observe contents covered with the gel and liquid test sample due to the characteristics of apparatuses such as a scanning electron microscope used to observe a surface of the test sample (several ten nanometers (nm)). Further, when test samples having a size of several tens to hundred nanometers are to be observed, the coating thickness has a significant influence on measurement errors. Accordingly, there is a need for overcoming the challenges of such technology.
To solve the above problems, a scanning electron microscope (hereinafter referred to as ‘air SEM’) capable of observing test samples in the air has been developed. Such an air SEM has an advantage in that, since there is an organic test sample in the air, the test sample can be observed without any additional pre-treatment since a charge-up effect becomes very poor as electrons accumulated on a surface of the test sample are neutralized with cations ionized from particles in the air when the surface of the test sample is irradiated with an electron beam.
However, the air SEM has a problem in that an electron beam may be dispersed in the air during a process of focusing the electron beam on a surface of the test sample left in the air. Therefore, to minimize a distance at which the electron beam is exposed to the air, the air SEM has a physical limitation in that a distance between the test sample and a shielding film, which is shot with an electron beam out of a vacuum state, is close to 50 to 200 μm to observe the test sample.
Accordingly, the air SEM has a problem in that it is primarily focused so that a surface of the test sample does not come in contact with the shielding film, and a slidable test sample stage is located while maintaining a constant height.